Traditional photographic objective designs, such as the Cooke triplet, typically have chief ray angles that are similar in magnitude to the field angle of the system. For example, U.S. Pat. No. 2,279,372 to Herzberger entitled, “Photographic Objective,” and U.S. Pat. No. 3,087,384 to Baur et al. entitled, “Triplet Wide-Angle Objective Lens,” both disclose Cooke-triplet type lens systems for photographic film applications. These prior art inventions both have relatively large chief ray angles because photographic film applications do not require reduced chief ray angles.
Today's photographic objectives may be used with small format digital image sensors whose resolution is typically between 640×480 pixels and 1280×960 pixels, where the pixels have dimensions less than 0.010 mm, as with cell phone camera applications. An imaging system for this type of application must have a full angular field of view between 50 and 70 degrees. In addition, the imaging system must meet size limitations, requiring chief ray incidence angles on the image plane to be limited to less than 20 degrees.
As digital imaging planes become smaller, the pixel count remains the same or even increases. A photographic objective lens may be developed for a new system by adapting a successful lens design for a 35 mm format by scaling the lens system according to the reduction in dimension of the image plane. For example, the diagonal of a 35 mm film frame is 43.3 mm, while the diagonal of a quarter-inch format has an image sensor diagonal of 5.6 mm. Thus, the lens system must be scaled down by 5.6/43.3 or approximately a factor of eight which means that the tolerance values must also be scaled down by the same amount in order to achieve the same image quality. This results in much tighter tolerance requirements for the scaled-down lens system, which are currently achievable in large scale manufacturing.
In the past, lens system assemblies designed as photographic objectives for small format image planes have had difficulty consistently meeting performance specifications due to the effects of manufacturing construction errors. Previous designs were too sensitive to alignment errors. Consequently, the sharpness of the image was reduced by an unacceptable amount when the strict centering requirements of the optical elements imposed by the design were not met.
The problem of achieving a reduced sensitivity to manufacturing tolerances in a wide-angle photographic objective lens system used with small format digital image sensors has not been adequately resolved.